Design a wake-up alarm app for iOS that "keeps ringing even after the user turns the phone off and on." The app doesn't auto-launch after a reboot and background execution is limited, so give me the most resilient design within those constraints.
Local notifications scheduled ahead of time stay with the system, so they fire after a reboot without the app running. Schedule one alarm as several notifications a few minutes apart from the start time, use the Critical Alert entitlement (requires approval) or Time Sensitive to get through silent and Focus modes, revalidate and reschedule whenever the app runs, and use a 30 second custom sound.
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